Processes to fabricate porous silicon and its use as feedstock for secondary battery electrodes

ABSTRACT

Provided are processes to form microporous silicon useful as an active material in an electrode of an electrochemical cell the processes including subjecting a mixture of silicon oxide and a metal reducing agent, optionally aluminum, to mechanical milling to form mechanically activated silicon oxide/aluminum, thermally treating the silicon oxide/aluminum to reduce the silicon oxide and form Si/Al 2 O 3 , and removing at least a portion of the alumina from the Si to form a microporous silicon. The resulting electrochemically active microporous silicon is also provided with residual alumina present at 15% by weight or less that demonstrates excellent cycle life and safety.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/741,903 filed Jan. 4, 2018, which is a U.S. National Phase under 35U.S.C. § 371 of PCT/US2016/041619 filed Jul. 8, 2016, and which dependsfrom and claims priority to U.S. Provisional Application No. 62/189,826filed Jul. 8, 2015, the entire contents of each of which areincorporated herein by reference.

FIELD

The invention relates generally to methods of producing anode activematerials. More specifically, methods are provided for the production ofmicroporous silicon that can be used as an anode active material insecondary batteries.

BACKGROUND

Microporous silicon has been widely produced in the semiconductingindustry for over 50 years. Electrochemical etching has been the mostinvestigated approach for the formation of microporous silicon forchip-based applications and has been used to create highly directionalmesoporosity and macroporosity. A number of techniques such asanodization, vapor etching, glancing angle deposition, lithographicetching, and photoetching are also suitable for Si wafer-basedprocessing. Other methods have been used with both wafer and powdersilicon feedstock, such as stain etching, galvanic etching, and metalnanoparticle-assisted etching.

There is an increasing need and interest in microporous silicon (μpSi)fabrication routes that utilize existing feedstock or silicon-basedmolecules that are themselves byproducts from silicon production. Suchprocesses typically use chemical conversion of, for example, silica,silane, or silicon tetrachloride [Bao Z, Weatherspoon M R, Shian S, CaiY, Graham P D, Allan S M, Ahmad G, Dickerson M B, Church B C, Kang Z,Abernathy H W III, Summers C J, Liu M, Sandhage K H (2007), Nat Lett446:172]. This chemical conversion can be promoted thermally,mechanically, or electrochemically. In these approaches the porosity andparticle morphology can be directed either by the starting solidfeedstock or by how the silicon nanoparticles assemble into a porousbody via a subsequent densification step.

Direct production of highly porous silicon by chemical conversion ofsilica is receiving recent attention for applications that requirerelatively inexpensive material. For example silica has been reduced tosilicon at moderate temperatures using magnesium vapor (magnesiothermicreduction) as a reducing agent with a range of different silicafeedstock, including diatoms, sand, opal, zeolite, aerogels, SBA-15,bamboo extract, template silica, rice husk ask, and silicon monoxide[Fukatani K, Ishida Y, Aiba T, Miyata H, Den T (2005), Appl Phys Lett87:253112, Krishnamurthy A, Rasmussen D H, Suni II (2011), J ElectrochemSoc 158(2):D68-D71]

The two most studied ways to produce μpSi powder are metal assistedetching of Si and magnesiothermic reduction of SiO₂. A greatdisadvantage to the etching approaches is the usage of the highlydangerous HF solution to create the pores on the silicon surfaces.Producing highly porous structures at high volumes via etchingtechniques typically generates large quantities of unreacted orunder-reacted solid silicon substrate as a waste product unless it isrecycled. In the case of the magnesiothermic reduction the majorchallenge is that the highly exothermic reaction would make it difficultto avoid sintering and collapse of the desired pore structure,particularly as the reaction is scaled up. A second challenge regardsapplications where porous silicon is required with minimum oxidecontent. Residual oxides may increase irreversible loss or initialcapacity loss (ICL) of lithium in the battery. The aqueous HCl atelevated temperatures used to remove the MgO by-product has oftenreintroduced significant amounts of silica (oxide) in highly poroussilicon. The use of aqueous HF as a final processing step to remove sucha phase increases the EHS burden of the magnesiothermic reductionprocess, especially when biogenic silicas are utilized [Batchelor L,Loni A, Canham L T, Hasan M, Coffer J L (2012), Silicon 4:259-266]

As such, new methods of producing microporous silicon are needed.

SUMMARY OF THE INVENTION

The following summary is provided to facilitate an understanding of someof the innovative features unique to the present disclosure and is notintended to be a full description. A full appreciation of the variousaspects of the disclosure can be gained by taking the entirespecification, claims, drawings, and abstract as a whole.

It is a first object to provide a process of forming anelectrochemically active silicon material using materials and processeswith lower environmental impact than prior methods. The processesprovided do not require harsh acids such as hydrogen fluoride asetchants and are capable of activation at relatively lower temperatures,e.g. less than 700° C. A process includes subjecting a combination ofpowdered silicon oxide and aluminum to mechanical milling to form amechanically activated silicon oxide/aluminum composite; thermallytreating said composite by exposure to heat at a temperature of 500degrees Celsius to 700 degrees Celsius under an inert or reducingatmosphere to form Si/Al₂O₃; and removing at least a portion of aluminafrom said Si/Al₂O₃ by exposing said Si/Al₂O₃ to an etchant to formmicroporous silicon. The precursor silicon oxide is optionally of anaverage linear dimension (e.g. diameter) of 500 μm or less. An advantageof the provided processes is their ability to reduce the silicon oxideat relatively lower temperatures. The heat used in a process itoptionally 500 degrees Celsius to 600 degrees Celsius. In the mechanicalmixing the ball to powder mass ratio is optionally 8:1 or greater. Othercharacteristics of the claimed processes are the ability to exclude theuse of etchants that include fluorine. As such, an etchant optionallyexcludes fluorine, optionally excludes hydrogen fluoride. An etchant asused in the processes optionally is or includes HCl, H₂SO₄, H₃PO₄, HNO₃or combinations thereof. In the processes, the weight percent of aluminaremoved is optionally 85 weight percent to 100 weight percent,optionally 85 weight percent to 99 weight percent. The subjecting stepoptionally includes high energy ball milling. A milling time isoptionally greater than 0.5 hours, optionally 0.5 to 24 hours. A step ofthermally treating is optionally for a thermal treatment time of 10minutes to 12 hours.

It is another object to provide a process of forming an electrodesuitable for use in an electrochemical cell. A process includes:providing an anode active material comprising microporous siliconoptionally produced as described herein by a two-step reduction process;combining said microporous silicon with a binder to form a slurry; andcoating said slurry on an electrically conductive substrate to form anelectrode. In some aspects, the active material further includesgraphite. Optionally the step of providing includes subjecting acombination of powdered silicon oxide and a metal reducing agent,optionally aluminum, to mechanical milling to form a mechanicallyactivated silicon oxide/aluminum composite, optionally SiO₂/Al, powder;thermally treating said silicon oxide/aluminum powder by exposure toheat at a temperature of 500 degrees Celsius to 700 degrees Celsiusunder an inert or reducing atmosphere to form Si/Al₂O₃; removing atleast a portion of alumina from said Si/Al₂O₃ by exposing said Si/Al₂O₃to an etchant to form microporous silicon. In some aspects, themicroporous silicon includes residual alumina at an alumina weightpercent of 0.1 to 15.

It is another object to provide an electrochemically active materialthat includes microporous silicon, optionally produced by a process asprovided herein. An electrochemically active material includes: amicroporous silicon and alumina, said alumina present at 15% or less byweight of said material and where the microporous silicon and thealumina are intermixed and mechanically activated. The alumina isoptionally present at 15% by weight or less, optionally 5% by weight orless, optionally 1% by weight or less. The electrochemically activematerial optionally further includes carbon. In some aspects, anelectrochemically active material is characterized by a cycle life of 80percent capacity or greater at cycle 40, optionally a cycle life of 80percent capacity or greater at cycle 80. In some aspects, anelectrochemically active material has an initial capacity loss of lessthan 20%. The alumina in an electrochemically active material optionallyincludes alpha-alumina, optionally includes gamma-alumina, or both. Insome aspects, an electrochemically active material optionally excludesalpha-alumina. In some aspects, an electrochemically active materialoptionally excludes gamma-alumina. In some aspects, an electrochemicallyactive material optionally excludes alpha-alumina and gamma-alumina.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary process;

FIG. 2A demonstrates X-ray diffraction patterns of milled and orthermally treated material according to some aspects with: trace (a) theinitial SiO₂ and Al mixture (premix at 2000 rpm with centrifugal mixer);trace (b) after mechanical milling activation (BPR 4: for 4 h); trace(c) unmilled powder (only premixed) after thermal treatment at 650° C.for 2 h (2.5° C./min heating ramp under argon atmosphere); and trace (d)mechanically milled material after final thermal reduction at 650° C.for 2 h (2.5° C./min heating ramp under argon atmosphere), (●) aluminum,(▪) silicon, (∘) α-alumina, (♦)γ-alumina, and (□) mullite([Al_(2.3)Si_(0.7)]O_(4.85));

FIG. 2B illustrates XRD patterns for powder mixtures mechanically milledat different conditions and thermally treated at the same temperature(●) Al metal, (▪) silicon, (♦) γ-Al₂O₃ (⋄) η-Al₂O₃(∘)α-Al₂O₃ (□) mullite([Al_(2.3)Si_(0.7)]O_(4.85));

FIG. 2C illustrates XRD patterns for powders thermally treated atdifferent temperatures and where mechanical milling conditions (e.g.BPR, etc.) were kept constant for all samples (●) Al metal, (▪) silicon,(♦) γ-Al₂O₃ (⋄) η-Al₂O₃ (∘)α-Al₂O₃ (□) Mullite([Al_(2.3)Si_(0.7)]O_(4.85));

FIG. 3A illustrates a SEM micrograph of a SiO₂ and Al mixture after ballmilling;

FIG. 3B illustrates a SEM micrograph of a microporous silicon fabricatedby the two-step reduction process and after 4-hour H₂SO₄ etching (no HFor metal catalyst);

FIG. 4 illustrates XRD patterns for samples after thermal treatment andexposure to an etchant under differing conditions illustrating residualoxides of less than 3% (●) Al metal, (▪) silicon, (∘)α-Al₂O₃ (□) mullite([Al_(2.3)Si_(0.7)]O_(4.85));

FIG. 5A illustrates the formation cycle of half-cell anode electrodeshand casted with μpSi and graphite (1:1 mass ratio) at a C/20 rate from0.01V to 2.0V, with ˜21% ICL and reversible capacity was 1600 mAh/g;

FIG. 5B illustrates rate capabilities of anodes based on commercial 1-5μm non-porous Si (□), conventional HF etched porous Si (⋄), and the μpSi(●) and natural graphite (1:1 mass ratio);

FIG. 5C illustrates cycle life comparison of various materials wherecycling performance for electrodes fabricated with commercial 1-5 μmnon-porous Si (□), conventional HF etched porous Si (⋄), and the μpSi(●) mixed with natural graphite (1:1 mass ratio); and

FIG. 6 illustrates differential scanning calorimetry plots for a (▬)porous Si/graphite electrode and (---) μpSi/Graphite composite eachfully lithiated (100% SOC).

DETAILED DESCRIPTION

The following description of particular aspect(s) is merely exemplary innature and is in no way intended to limit the scope of the invention,its application, or uses, which may, of course, vary. The invention isdescribed with relation to the non-limiting definitions and terminologyincluded herein. These definitions and terminology are not designed tofunction as a limitation on the scope or practice of the invention butare presented for illustrative and descriptive purposes only. While theprocesses or compositions are described as an order of individual stepsor using specific materials, it is appreciated that steps or materialsmay be interchangeable such that the description of the invention mayinclude multiple parts or steps arranged in many ways as is readilyappreciated by one of skill in the art.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers, and/or sections, these elements,components, regions, layers, and/or sections should not be limited bythese terms. These terms are only used to distinguish one element,component, region, layer, or section from another element, component,region, layer, or section. Thus, “a first element,” “component,”“region,” “layer,” or “section” discussed below could be termed a second(or other) element, component, region, layer, or section withoutdeparting from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “Or” means “and/or.” As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof. The term “or a combination thereof” means a combinationincluding at least one of the foregoing elements.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

As used herein, the term “microporous” with respect to microporoussilicon is defined as having an average particle size of 0.1 to 50micrometers (μm) and an average pore diameter of 20 to 500 nanometers(nm). Microporous silicon may have a raspberry-like overall structure.

As used herein, the term “high energy” is defined as milling with energyat or above that produced by a Retsch PM 400 planetary mill with arational speed of equal to or greater than 250 revolutions per minute(rpm) with a ball to powder ratio of equal to or greater than 10:1.

As used herein, the term “low energy” is defined as milling with energyat or below that produced by a Retsch PM 400 planetary mill with arational speed of equal to or lower than 200 revolutions per minute(rpm) with a ball to powder mass ratio of less than 10:1.

Microporous silicon structures have advantages over conventional siliconparticles in lithium ion battery (LIB) anodes. The empty (void) space ofthe pores present in microporous silicon improves the electrode cyclelife. This can be possible because the free space accommodates stressescreated due to expansion and contraction occurring duringlithiation/delithiation of the anode. The present disclosure providedprocesses of forming microporous silicon that show improvedelectrochemical properties and do not require harsh etchants such asfluorine containing etchants.

Provided are processes of forming microporous silicon suitable for useas an electrochemically active material, optionally in an anode, inelectrochemical cells, as a supplement to an electrochemically activematerial, or as an electrochemically active material along with a secondelectrochemically active material. The processes provided combine a highenergy milling process with a thermal reduction to produce solid siliconsubstantially by the overall reaction according to equation 1:

$\begin{matrix}\left. {{3{{SiO}_{2}(s)}} + {4{{Al}(l)}}}\rightarrow{{3{{Si}(s)}} + {3{Al}_{2}{O_{2}(s)}}} \right. & (1)\end{matrix}$

A process includes subjecting a combination of silicon oxide, optionallypowdered silicon oxide, and aluminum to mechanical milling to form amechanically activated silicon oxide/aluminum, optionally SiO₂/Al,powder, thermally treating the silicon oxide/aluminum powder by exposureto heat at a temperature of 500 degrees Celsius to 1100 degrees Celsius,optionally 500 degrees Celsius to 700 degrees Celsius, under an inert orreducing atmosphere to form Si/Al₂O₃, and removing at least a portion ofalumina from the Si/Al₂O₃ by exposing the Si/Al₂O₃ to an etchant to formmicroporous silicon optionally with average pore size ranging from10-500 nm, optionally about 20 nm to 200 nm, optionally about 150 nm.The provided processes do not require the use of etchants that includeharsh acids such as HF as etchants. As such, etching or other reactionwith a fluorine containing etchant such as hydrogen fluoride (HF),buffered HF, NH₄F or HF/HNO₃ mixtures are avoided.

Although the present disclosure is directed to aluminum as a reducingmetal, other metals may be substituted therefore with other reducingmetals illustratively including magnesium, calcium, aluminum silicide(AlSi₂), magnesium silicide (Mg₂Si), calcium silicide (Ca₂Si) or acombination thereof. In some aspects a metal reducing agent is aluminumexclusive of other reducing metals, a process optionally excludingmagnesium, calcium, aluminum silicide (AlSi₂), magnesium silicide(Mg₂Si), calcium silicide (Ca₂Si) or a combination thereof.

An exemplary process is illustrated in the schematic of FIG. 1. At step102, the starting materials are industrial grade (e.g. >98% pure orother industrial feedstock rather than solar or electronic grade as arerecognized in the art) such as micron-size silicon oxide or silica, andmicron-size aluminum metal (or other metal reducing agent) powders. Thesilicon containing material and aluminum average particle sizes areoptionally about 20 and about 17 microns, respectively. However, largerparticle size up to −325 mesh (˜45 microns) can be used for both theoxide and the metal powders. Such powders are commercially available andcommonly produced; illustratively from Alfa Aesar. The powders (e.g. SiOor SiO₂, and Al) are loaded into a reaction chamber at a desired massratio. A mass ratio is optionally a stoichiometric mass ratio based onequation (1). In some aspects a mass ratio includes excess reducingmetal to obtain a completed or more completed reaction; optionally amass ratio includes up to 40% extra reducing metal. A reaction chamberis optionally any container suitable for high energy milling, thermaltreating, or other and optionally sealable from the externalenvironment. An illustrative example of a reaction chamber is astainless steel or ceramic jar. Included in the reaction chamber ismilling media optionally formed of the same material as the reactionchamber. In some aspects, the milling media is or includes stainlesssteel or ceramic milling balls optionally ½ inch in diameter.

The components are subjected to a mechanical milling process such asball milling, impact milling, attritor milling, and the like. Themechanical milling step provides mechanical activation and partialreduction of the silicon oxide precursor (combined described herein as“activation”). The mechanical activation of the material by millingfacilitates the reduction reaction through both reducing the reactantdiffusion distance and introducing defects and other forms of residualmechanical strain in the material. The temperature of chemical reductionof SiO₂ can be reduced to <800° C. when mechanical activation isemployed, which otherwise requires T>1000° C. Mechanical millingprovides a means to control the reaction kinetics of the reductionreactions. As a consequence, reactions that normally require hightemperatures will occur at lower temperatures. In addition, the highdefect densities induced by milling accelerate the diffusion process. Inthe example of FIG. 1 at 104, the powder mixture is milled using a highenergy ball mill. This step produces the mechanical activation andpartial displacement reduction reaction according to Equation 1.Repeated welding and fracturing of powder particles during themechanical milling increases the area of contact between the reactantpowder particles due to a reduction in particle size and allows freshsurfaces to come into contact repeatedly. The ball milling facilitatesthe reduction reaction through both reducing the reactant diffusiondistance and introducing defects and other forms of residual mechanicalstrain. As a consequence, subsequent thermal treatment reactions thatwould otherwise require high temperatures will occur at lowertemperatures during high energy milling without any externally appliedheat. In addition, the high defect densities induced by millingaccelerate the diffusion process. A high energy planetary ball mill(e.g. Retsch PM400) may be used to perform the mechanical millingreactions.

Mechanical milling is optionally achieved by ball milling, optionallyhigh energy ball milling. A ball to powder (e.g. Si containing materialand aluminum) mass ratio (BPR) may be 4:1 or greater, optionally 5:1,optionally 6:1, optionally 7:1, optionally 8:1, optionally 9:1,optionally 10:1, optionally 11:1, optionally 12:1, optionally 13:1,optionally 14:1, optionally 15:1, optionally 16:1, or greater. In someaspects, the BPR is 8:1 or greater. In some aspects, the BPR is 16:1 orgreater. Optionally the BPR is from 4:1 to less than 16:1. Optionallythe BPR is from 8:1 to less than 16:1. Varying the ball to powder ratio(at the same milling speed and time, and same reactant ratio) willaffect the product obtained after thermal treatment. For example, whenmaterials were thermally reduced at the same conditions (650° C. for 2h, 2.5° C./min heating ramp under argon atmosphere), at 4:1 BPR for amilling reaction time of 4 h the main alumina phase is α-Al₂O₃ withsmall amount of γ phase and mullite ([Al_(2.3)Si_(0.7)]O_(4.85)). As theBPR increases to 8:1 (same other reaction conditions) γ-Al₂O₃ is theresulting he main alumina phase. By increasing BPR to 16:1 other lessstable Al₂O₃ phases appear (η, β, δ, etc.).

Mechanical milling is performed for a milling reaction time. A millingreaction time is any time suitable to activate the powders and at leastpartially reduce the silica. A milling reaction time is optionally from0.2 hours to 20 hours, or any value or range therebetween, optionally0.2 hours to 10 hours, optionally 0.2 hours to 5 hours, optionally 0.5hours.

Milling is optionally performed under a non-reactive atmosphere such asan inert gas or a reducing atmosphere. Non-reactive atmospheres areknown in the art, illustratively argon, xenon, or others. Adjustablemechanical milling parameters include rotational speed (or cycles/min),ball:powder mass ratio (BPR), ball size, silica:aluminum mass ratio. Insome aspects, the mechanical milling may be performed at the followingconditions, 1200 cycles/min, with 4:1 BPR, ½ inch diameter stainlesssteel balls and a stoichiometric 5:3 silica:aluminum mass ratio. Therevolutions per minute or number of cycles per minute can vary between200 and 1200; ball to powder ratios can fluctuate from 4:1 to 20:1;different media size from 5 to 15 mm can be used as well as othermaterials such as hardened steel, zirconia, etc. To ensure fullreduction of silica, the aluminum quantity can be increased from theideal (stoichiometric) value up to stoichiometric 40% excess, optionallyup to stoichiometric 10% excess.

The mechanically activated SiO₂/Al powder is then thermally treated atstep 106 in FIG. 1 in a tube (or other) furnace under an inert gasenvironment. Thermal reduction of silicon oxide by aluminothermicreaction initiates at temperatures below the aluminum melting point(660.9° C.). At temperatures as low as 500° C. reduction to silicon fromsilica and the formation of Si/Al₂O₃ begins to take place in the solidstate. However, in the absence of mechanical milling the reductionreaction needs to reach temperatures >800° C. (to ˜1100° C.) for thecomplete transformation of SiO₂/Al to Si/Al₂O₃. The step of mechanicalmilling significantly reduces this temperature requirement wherecomplete conversion to Si/Al₂O₃ can be achieved at temperatures below700° C. Thermal reduction is performed using a thermal treatmenttemperature for a reduction time.

A thermal treatment temperature is optionally from 500° C. to 1100° C.,or any value or range therebetween. In some aspects, a thermal reductiontemperature is 500° C. to 700° C., optionally 650° C. Optionally, athermal reduction temperature is 500° C. to 650° C., optionally 525° C.to 700° C., optionally 525° C. to 650° C., optionally 500° C. to 600°C., optionally 600° C. to 700° C., optionally 600° C. to 650° C. In someaspects, a thermal reduction temperature does not exceed 700° C.,optionally does not exceed 650° C., optionally does not exceed 600° C.XRD patterns show that the milled powder mixture should be heated up toat least 500° C. to initiate the SiO₂ reduction. And at ˜550° C. all theAl will be consumed in the reaction in the production of Si/Al₂O₃. From500° C. to less than 600° C. the main oxide phase is γ-Al₂O₃. As thetemperature increases the γ-phase transforms to the more stable α-Al₂O₃phase.

The resulting alumina produced is optionally less than 50% alpha phase,optionally less than 25% alpha phase, optionally less than 10% alphaphase, optionally less than 5% alpha, optionally less than 1% alphaphase relative to the total alumina present

A reduction time is optionally about 1 to about 4 hours, or any value orrange therebetween. In some aspects, a reduction time is about 0.5 toabout 2 hours, optionally about 2 hours. During a thermal reduction timea temperature increase is optionally a 2° C./min to 2.5° C./min heatingrate. The thermal reduction products are elemental Si and aluminum oxide(Al₂O₃, also known as alumina). Adjustable thermal reduction parametersinclude heating rate, holding temperature and time. The heating ratesare optionally kept between 0.5° C./min and 5° C./min and using theholding temperature and time ranges aforementioned.

The last step to synthesize μpSi is the removal of alumina by etching.Etching optionally excludes subjecting the material to contact with aharsh acid such as hydrogen fluoride or other fluorine containingetchant. The step of etching depicted in FIG. 1 at 108 optionally usesetchant(s) that do not result in the production of toxic waste. Theetching process optionally will yield μpSi with the removal of aluminaby dissolution. Illustrative etchants include particular acids,illustratively HCl, H₂SO₄, H₃PO₄, HNO₃, or combinations thereof. Aconcentration of etchant is optionally 0.5-12 M, optionally 0.5 to 4 M,optionally at or about 9 M, in the case of H₂SO₄, and similarly reactiveconcentrations for the use of other etchants. Etching is performed foran etching time, optionally from 1 to 6 hours. Etching is performed atan etching temperature. Etching temperature is optionally 25° C. to 250°C. or any value or range therebetween, optionally 80° C. or 250° C.,optionally 80° C. to 140° C., optionally 180° C.

Aluminum oxide exists in a number of different phases (α, β, γ, η, δ, θ,κand ρ). α-Al₂O₃ is the most stable and least reactive phase. Oneadvantage of the milling process is that the subsequent thermalreduction may be designed to limit formation of α-Al₂O₃, and limit theneed for more aggressive dissolution in the etching phase. Other phasessuch as γ-Al₂O₃ and θ-Al₂O₃ can be dissolved in dilute hydrochloric acid(HCl) at relatively low temperatures.

Etching time and temperature are related to the degree of etchingdesired. Etching time is optionally from 1 to 3 hours, optionally 2hours. As the etching proceeds the pores on the silicon particle growincreasing particle porosity and BET surface area. In addition,increased etching time decreases the amount of alumina remaining on theparticles. It is believed that some residual Al₂O₃ may be tolerable oreven beneficial in the final product as this phase is electrochemicallyinactive and may provide beneficial structural reinforcement of the μpSiparticles. It is further believed that a modest decrease in capacity(mAh/g) due to the residual α-Al₂O₃ may be more than compensated byincreased cycle life of the nanocomposite anode material. As such, theamount of residual alumina, optionally α-alumina, is optionally 15% orless, optionally 10% or less, optionally 5% or less, optionally 3% orless, optionally 2% or less with percentages being weight percentrelative to the amount of alumina prior to etching. Residual alumina,optionally α-alumina, is optionally from 0.1% to 15%, optionally 0.1% to10%, optionally 0.1% to 5%, optionally 0.1% to 3%, optionally 0.1% to 2%with percentages being weight percent relative to the amount of aluminaprior to etching.

Following the etching process the resulting μpSi has a desired porosity.In some aspects, the porosity is 10% to 90% or any value or rangetherebetween, optionally 20% to 80%, optionally 30% to 60%, optionallyat or about 50%.

The resulting μpSi is optionally tailored for a desiredBrunauer-Emmett-Teller (BET) surface area. A BET surface area isoptionally 10 m²/g to 500 m²/g or any value or range therebetween,optionally 20 m²/g to 200 m²/g, optionally 20 m²/g to 100 m²/g.

The resulting μpSi is optionally dried and sieved to a desired size.Drying may be performed in a standard drying oven used for electrodematerials known in the art. Drying may be performed under an inert orair atmosphere at a temperature of 80° C. or other desired temperature.

The resulting particles are optionally sieved to a desired size.Illustrative particle sizes have an average diameter of 0.5 to 10microns. A μpSi prior to combination with a carbon, a binder, or bothmay be in any physical form such as a particulate (e.g. powder),nanowire, sheet, nanotube, nanofiber, porous structure, whisker,nanoplatelet, or other configuration known in the art.

The μpSi has a porosity, optionally with an average pore size of at orabout 10 nm to at or about 500 nm, optionally at or about 100 nm to ator about 200 nm, optionally at or about 150 nm.

The μpSi is optionally used as an active material alone or incombination with one or more other active materials in the formation ofan electrode for use in a primary or secondary battery. The μpSi isoptionally combined with a second active material, optionally carbon orgraphitized carbon. Carbon and graphitic carbon materials such asnatural graphite, graphene, artificial graphite, expanded graphite,carbon fibers, hard carbon, carbon black, carbon nanotubes, fullerenes,and activated carbon may be used. In some aspects, μpSi powders arecombined with graphite. Low energy ball milling may be used to create agraphite shell around silicon particles. In order to guarantee goodelectrode coating and cell performance, the μpSi/Graphite composite mayhave the following properties: 5-10 μm particle size, a BET surface areaof less than 20 m²/g, and an anode tap density of 0-8-1.2 g/cm³. Theseproperties will allow fabrication of coatings with electrode loadings ofgreater than 3 mAh/cm².

A μpSi active material may or may not be associated with an electricallyconductive substrate. When associated with a substrate, the substrate isoptionally formed of any suitable electronically conductive andimpermeable or substantially impermeable material, including, but notlimited to, copper, stainless steel, titanium, or carbon papers/films, anon-perforated metal foil, aluminum foil, cladding material includingnickel and aluminum, cladding material including copper and aluminum,nickel plated steel, nickel plated copper, nickel plated aluminum, gold,silver, any other suitable electronically conductive and impermeablematerial or any suitable combination thereof. In some aspects,substrates may be formed of one or more suitable metals or combinationof metals (e.g., alloys, solid solutions, plated metals). Optionally, aμpSi active material is not associated with a substrate.

A μpSi active material may be associated with a binder. A bindermaterial is optionally used at 1-10% by weight of solvent and combinedwith the μpSi active material. A binder material optionally includescommon Si anode binder such as carboxymethyl cellulose (CMC) orstyrene-butadiene rubber (SBR) binders. In some aspects PVdF bindersolutions in NMP or aqueous polyolefin latex suspensions may be used.Examples of the solvent used in preparation of the electrode mayinclude, but are not limited to aqueous, carbonate-based, ester-based,ether-based, ketone-based, alcohol-based, or aprotic solvents. Specificsolvents such as dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP)and ethylene glycol, and distilled water may be used. Such solvents areknown in the art. The resulting slurry may be applied to the substratefollowed by standard techniques including casting, drying, andoptionally pressing. The substrate is optionally electrically associatedwith an electrode tab in order to electrically connect the electrode tothe appropriate terminal of a battery (i.e., negative electrode tonegative terminal and positive electrode to positive terminal). Theelectrode tab may be welded by a method of welding that includes, but isnot limited to, resistance welding, laser welding, electron beamwelding, or ultrasonic welding.

An electrochemical cell typically includes a separator positionedbetween the electrodes. A separator is optionally a non-woven, felted,nylon, or polypropylene material that is permeable to hydroxide ions andmay be suitably saturated with electrolyte.

An electrochemically active material including the μpSi as providedherein alone or in combination with another active, optionally a carbonactive, shows improved cycle life and similar initial capacity loss(ICL) relative to other silicon electrochemically active materials. AμpSi optionally has a cycle life of greater than 80% residual capacityat cycle 40, cycle 45, cycle 50, cycle 55, cycle 60, cycle 65, cycle 70,cycle 75, cycle 80, cycle 85, cycle 90, cycle 95, or greater. Priormaterials measured under the same conditions show significantly lowercycle life as defined as the cycle which the capacity drops below 80%residual capacity.

An important characteristic of the μpSi materials as provided herein arethat the ICL is nearly equivalent to other materials such as otherporous silicon materials, but shows significantly improved cycle life.As such, the ICL of a μpSi as provided herein is optionally from 15% to25%, optionally from 19% to 21%, optionally less than 20%.

An anode, cathode, electrolyte, and separator may be housed in a casingas is typically known in the art to form an enclosed or substantiallyenclosed electrochemical cell.

Various aspects of the present invention are illustrated by thefollowing non-limiting examples. The examples are for illustrativepurposes and are not a limitation on any practice of the presentinvention. It will be understood that variations and modifications canbe made without departing from the spirit and scope of the invention.

EXAMPLES

Inexpensive commercially available micron size silica (SiO₂) powder(Grace Davison PERKASIL Silica, 15-20 μm, BET Surface areas 190 m²/g,98% SiO₂ dry basis) (Other silica used, U.S. Silica MIN-USIL, 5-45micron particle size, 99.4% SiO₂) was pre-mixed (FlackTek Speed mixer,2000 rpm for 2 mins three times and stirred with a spatula) withaluminum powder (mesh 325, 99.97% Al, Alfa Aesar) and transferred to ahardened steel jar along with milling media of the same composition. Thejar was filled with argon and sealed before mechanical milling.High-energy ball milling (stoichiometric mass ratio of reactants, 4:1ball to powder ratio, milled for 4 h hours using SPEX 8000, 1200-1500cycles per minute) activated the powder systems and partially reducedthe silica as milling proceeded (t>0.5 h). After mechanical milling, thepowder mixture was transferred to a tube furnace to proceed with thethermal reduction. Thermal reduction was performed over a range oftemperatures from 500° C. to 700° C. under argon atmosphere. XRDpatterns taken at different stages of the reduction process are shown inFIG. 2A. The initial powder mixture shows strong aluminum XRD peaks andalmost unnoticeable silica broad peaks. After 0.5-4 h time periods ofhigh energy ball milling small peaks of alumina start to appear on thepattern. Thermal treatment at 650° C. for 2 hours (with a 2.5° C./minheating rate) led to the complete conversion SiO₂/Al to Si/Al₂O₃ asshown in the pattern (top line) in FIG. 2A.

Process parameters such as BPR affect the resulting type of aluminaformed in the process. Samples prepared as above (pre-mixed using aFlackTek Speed mixer, 2000 rpm 2 mins three times and stir with spatula,stoichiometric reactants mass ratio) and thermally reduced at the sameconditions (650° C. for 2 h, 2.5° C./min heating ramp under argonatmosphere), but milled at various BPR ratios for 4 hours (using SPEX8000, 1200-1500 cycles per minute) demonstrate differing phases ofalumina as illustrated in FIG. 2B. At 4:1 BPR for 4 h majority of Al₂O₃is in the alpha (α) phase, with small amounts of gamma (γ) and Mullite([Al_(2.3)Si_(0.7)]O_(4.85)) present. As the BPR increases to 8:1 (samemilling time, etc.) γ-Al₂O₃ becomes the most prominent phase. IncreasingBPR to 16:1 produces other less stable Al₂O₃ phases (η, β, δ, etc.).

Thermal treatment temperatures also play a role in the type of aluminaproduced by the processes. FIG. 2C illustrates XRD patterns for milledpowder samples treated at different thermal reduction temperatures (450°C., 500° C., 525° C., 550° C., 600° C. and 650° C.). All samples wereball milled at BPR 4:1 for 2 h (at the same milling speed and time, andsame stoichiometric reactant ratio). The holding time was 2 h with a2.5° C./min heating ramp under argon atmosphere. Si (reduced) peakstarts to appear at ˜500° C. and it is fully reduced at T≥525° C. From500° C. to less than 600° C. the main oxide phase is the γ-Al₂O₃ phase.As the temperature increases the γ-phase transforms to the more stableα-Al₂O₃ is the main phase.

The final Si/Al₂O₃ powder mixtures were then treated with an etchantoptionally including an acid (e.g. HCl, H₂SO₄). By avoiding the use ofHF or other harsh etchants, less costly reaction vessels and othersafety precautions (e.g. hood use) are less demanding due to theincreased safety associated with the presently provided processes. Theetching process was carried out at 180° C. for 4 h under continuousstirring; this process removed most of the alumina (EDS spectra shows ˜5wt % elemental Al left) leaving Si particles with a porous structure.FIG. 3 shows SEM micrographs of the as thermal treated powder (A) andthe porous structure after 4 h of etchant digestion (B, 60 wt % sulfuricacid). XRD data (FIG. 4) indicate the main component of the thermallytreated powder is elemental Si with small traces of alumina (broad shortpeaks at 24-26 and 43-45 diffraction angles). FIG. 4 shows diffractionpatterns for two acid etching conditions, condition 1 was done with 70%H₂SO₄ for 1 h, and condition 2 was done with 70% H₂SO₄ for 4 h. Thephysical properties of the microporous silicon are illustrated in Table1.

TABLE 1 mPSi with Property Commercial Si (>97% Si) Total Pore Area(m²/g) 8.6 11.5 Average Pore Diameter (nm) 310 150 Bulk Density (g/mL)0.81 0.84 Skeletal density (g/mL) 1.78 1.34 BET surface area (m²/g) 4.336.2

Electrochemical properties of μpSi and the influence of the porousstructure on LiB anode performance were evaluated in half coin cells.Electrodes were coated using a combination of μpSi and commercialgraphite (Nippon Carbon Co. natural graphite, AZBD series, tap density1.01 g/cm³, BET surface area 2.71 m²/g, D₅₀ particle size 14.2 microns).Commercial micron size Si (silicon powder, 1-5 micron, 99.9%, AlfaAesar) before and after HF etching (HF etched μpSi, 6M HF etched for 8h) were used for comparison. To form the μpSi containing electrodes,graphite and porous silicon were mechanically (100-1200 rpm, Retsch PM400 planetary ball mill) mixed in a 1:1 mass ratio. Both μpSi/graphiteand graphite electrodes were hand casted 10 μm copper foil using waterbased binders at a mass ratio of 92/1.5/6.5 for silicon/graphite/bindermaterial respectively. The materials were tested in CR2025 half cells.The half-cell formation was carried out at C/20 current rate from 10 mVto 1.5V. Rate capability and cycle life were performed with an operationvoltage of 0.01-0.7V. For rate studies cells were lithiated at C/10 anddelithiated at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0C. Cycling experimentswere run at 0.5C.

Table 2 summarizes first formation cycle values for tested cells, theelectrochemical loading was ˜2 mg/cm². Initial lithiation capacity islower for the μpSi anode formed using the material made by the two-stepactivation process as follows. The μpSi was made by ball milling 15-20micron silica (Grace Davison) and mesh 325 Al (Alfa Aesar) at 4:1 BPRfor 4 hour using a stoichiometric reactant mass ratio; the milled powderwas thermally treated at 600° C. under Ar, for 2 h using a 2° C./minheating rate; thermally treated precuts were etched using 40% H₂SO₄ for4 h. A graphite shell was added to the μpSi particle by ball millinggraphite (Nippon Carbon Co. natural graphite, AZBD series, tap density1.01 g/cm³, BET surface area 2.71 m²/g, D₅₀ particle size 14.2 microns)with μpSi at low energy (100-1200 rpm for 4 h, Retsch PM 400 planetaryball mill). The difference in capacity is believed to be due to the 5%metal oxide (calculated using EDS and XRD data) content in the activematerial.

TABLE 2 1st Cycle Specific Capacity (mAh/mg) ICL Anode Active MaterialLithiation Delithiation (%) Si-Graphite Commercial μsize Si 1781 1441 19composite HF etched μpSi 1795 1436 20 μpSi 1671 1357 19

FIG. 5A shows the first formation cycle (at C/20 rate) for theμpSi/graphite composite half-cell anode illustrating the reversiblecapacity ˜1600 mAh/g. Rate performance and cycle life experiments werealso performed.

Rate capability was tested using cells lithiated at C/10 and delithiatedat 0.1C, 0.2C, 0.5C, 1.0C, 2.0C and 5.0C. Tests show good behavior ofμpSi/graphite comparable to the pure graphite electrodes as illustratedin FIG. 5B.

In addition, cycle life testing results show a clear advantage whenusing μpSi which has a much improved cycle life over the non-porousmicron sized Si. The processed μpSi/metal oxide using the proceduresdescribed above to produce μpSi also shows a noticeably higher capacityretention than HF-etched porous Si, which suggests a combined benefit ofthe porous structure and the stabilization effect of metal oxide shownin FIG. 5C. Without being limited to one particular theory, the empty(void) space of the μpSi pores is believed to improve the electrodecycle life. This free space is believed to accommodate stresses createddue to expansion and contraction occurring duringlithiation/delithiation of the anode.

Safety and abuse tolerance of the Si composite anodes were performed bydifferential scanning calorimetry (DSC). DSC enables the thermalresponse of individual and selected combinations of cell components tobe measured over a broad temperature range. DSC allows qualitativemeasurements of the local charge state of the electrodes, which impactsthe cell thermal reactivity leading to cell thermal runaway as well ascell self-discharge.

DSC measurements were made for porous Si (HF etched Si) and μpSi (madeby the processes as follows) with composites with different contents ofalumina. The μpSi was made by ball milling 15-20 micron silica (GraceDavison) and mesh 325 Al (Alfa Aesar) at 4:1 BPR for 4 hour using astoichiometric reactant mass ratio; the milled powder was thermallytreated at 600° C. under Ar, for 2 h using a 2° C./min heating rate;thermally treated precuts were etched using 40% H₂SO₄ for 4 h. Thedifferent porous materials were mixed with natural graphite at a 1:1weight ratio by ball milling graphite (Nippon Carbon Co. naturalgraphite, AZBD series, tap density 1.01 g/cm³, BET surface area 2.71m²/g, D₅₀ particle size 14.2 microns) with the μpSi at low energy(100-1200 rpm for 4 h, Retsch PM 400 planetary ball mill). Safetystudies were performed on fully lithiated (100% SOC) electrodes.Measurements were made on disassembled cells as well as laboratoryhalf-cells enabling detailed characterization of individual electrodechanges under controlled conditions. After formation, the cells weretaken to full charge and disassembled inside an argon-filled glovebox,the anodes were harvested and transferred and hermetically sealed in DSCpans. The sealed pan was transferred to a TA DSC Q200 instrument for DSCanalysis. DSC measurements were performed under N₂ at 30° C.-400° C.with a heating rate of 5° C./min.

DSC curves for two anode materials are shown FIG. 6. The anodes comparedwere HF etched porous Si/graphite (baseline) and μpSi/graphite formed asabove. The HF etched porous Si was made using 1-5 micron Si powder(99.9%, Alfa Aesar) and 6M HF solution, the reaction was held for 8 h.The μpSi/made by ball milling 15-20 micron Silica (Grace Davison) andmesh 325 Al (Alfa Aesar) at 4:1 BPR for 4 hour using a stoichiometricreactant mass ratio; the milled powder was thermally treated at 600° C.under Ar, for 2 h using a 2° C./min heating rate; thermally treatedprecuts were etched using 40% H₂SO₄ for 4 h. The addition of graphite tomake porous Si/graphite and μpSi/graphite was done by ball millinggraphite (Nippon Carbon Co. natural graphite, AZBD series, tap density1.01 g/cm³, BET surface area 2.71 m²/g, D₅₀ particle size 14.2 microns)with porous Si or μpSi /Al₂O₃; at low energy (100-1200 rpm for 4 h,Retsch PM 400 planetary ball mill). Both anodes showed exothermscommonly attributed to reaction of the solid electrolyte interface (SEI)layer starting around 120° C. but the baseline anode clearly liberatedmore heat and at lower temperatures. Additionally, the experiments ongraphite anodes at the same conditions run by Sandia National Lab[Doughty 2012], showed >2× higher heat release than the Si/graphite(baseline). These results confirm that the improved electrochemical andthermal properties are attributable to the novel architecture of ourproposed anode material.

Various modifications of the present invention, in addition to thoseshown and described herein, will be apparent to those skilled in the artof the above description. Such modifications are also intended to fallwithin the scope of the appended claims.

It is appreciated that all reagents are obtainable by sources known inthe art unless otherwise specified.

REFERENCE LIST

Zhang, P., et al. U.S. Pat. No. 7,722,991 B2 May 25, 2010

Richard, M., et al., U.S. Pat. No. 8,039,152 B2 Oct. 18, 2011

Zhang, P., et al., U.S. Pat. No. 8,679,679 B2 Mar. 25, 2014

Mao, O. et al., Electrochem. Solid-State Lett. 1999, 2, 3-5.

Obrovac, M., et al., J. Electrochem. Soc. 2007, 154, 9, A849-A855.

http://www2.lbl.gov/dir/assets/docs/TRL % 20guide.pdf

Standage, A., et al., J Am. Ceram. Soc. 1967, 50(2), 101-105.

Dequing W., et al., J. Mater. Synthesis and Process. 2012, 9, 5-9

Grigoryeva, T., et al., J. of Physics: Conference Series 2009, 144,021080.

Zheng, Y. et al., Electrochim. Acta 2007, 52, 5863-5867.

Matteazzi, P., et al., Amer. Ceram. Soc. 1992, 75, 2749-55.

He, Y., et al., Adv. Mater. 2011, 23, 4938-4941.

Hwang, G., et al., Chem. Commun. 2015, 51, 4429-4432.

Wang, J., et al., J. Mater. Chem. A. 2014, 2, 2306-2312.

Nguyen, H., et al., J. Mater. Chem., 2012, 22, 24618-24626.

Kim, S., et al., J. Mater. Chem. A, 2015, 3, 2399-2406.

Gallagher, K., Ener. Environmental Sci. 2014, 7 (5), 1555-1563.

Manufacturing Readiness Level (MRL) Deskbok, DOD, ODS ManufacturingTechnology Program, Version 2.0,

Suryanarayana, C. Progress in Materials Science, 2001, 46, 1-184.

Prabriputaloong, K. and Piggott, M., J. Electrochem. Soc.: Solid-StateSci. Tech. 1974, 121, 3.

http://www.chemguide.co.uk/inorganic/period3/oxidesh2o.html, Apr. 5,2015

Patents, publications, and applications mentioned in the specificationare indicative of the levels of those skilled in the art to which theinvention pertains. These patents, publications, and applications areincorporated herein by reference to the same extent as if eachindividual patent, publication, or application was specifically andindividually incorporated herein by reference.

The foregoing description is illustrative of particular embodiments ofthe invention, but is not meant to be a limitation upon the practicethereof.

1. A process for the formation of microporous silicon according to claim17 comprising: subjecting a combination of powdered silicon oxide andaluminum to mechanical milling to form a mechanically activated siliconoxide/aluminum; thermally treating said silicon oxide/aluminum byexposure to heat at a temperature of 500 degrees Celsius to 650 degreesCelsius for 4 hours or less under an inert or reducing atmosphere toform Si/Al₂O₃ such that said Si/Al₂O₃ comprises alumina with apredominant γ-Al₂O₃ phase; and removing at least a portion of aluminafrom said Si/Al₂O₃ by exposing said Si/Al₂O₃ to an etchant to formmicroporous silicon; wherein said microporous silicon further comprises15% or less by weight alumina.
 2. The process of claim 1 wherein saidsilicon oxide has a linear dimension of 500 micrometers or less.
 3. Theprocess of claim 1 wherein said heat is at a temperature of 500 degreesCelsius to 600 degrees Celsius.
 4. The process of claim 1 wherein saidstep of subjecting is by ball milling at a ball to powder mass ratio of4:1 to 16:1.
 5. The process of claim 1 wherein said etchant excludesfluorine.
 6. The process of claim 1 wherein said etchant excludeshydrogen fluoride.
 7. The process of claim 1 wherein 85 alumina weightpercent to 100 alumina weight percent of said alumina is removed by saidstep of removing.
 8. The process of any claim 1 wherein 85 aluminaweight percent to 99 alumina weight percent alumina is removed by saidstep of removing.
 9. The process of claim 1 wherein said acid comprisesHCl, H₂SO₄, H₃PO₄, HNO₃ or combinations thereof.
 10. The process ofclaim 1 wherein said step of subjecting is in a high energy ball mill.11. The process of claim 1 wherein said step of subjecting is for amilling time of 0.5 to 24 hours.
 12. An electrochemically activematerial comprising: a microporous silicon, wherein said microporoussilicon is formed by a process comprising mechanically milling acombination of powdered silicon oxide and aluminum and subsequentlythermally treating said silicon oxide and aluminum to heat at atemperature of less than 700 degrees Celsius for 4 hours or less; andalumina present with a γ-Al₂O₃ phase, said alumina present at 15% orless by weight of said material; said microporous silicon and saidalumina intermixed and mechanically activated.
 13. The electrochemicallyactive material of claim 12, said alumina present at 5% by weight orless.
 14. The electrochemically active material of claim 12 furthercomprising carbon.
 15. The electrochemically active material of claim12, said alumina further comprising alpha-alumina.
 16. Theelectrochemically active material of claim 12 having a porosity of 10%to 90%.
 17. The electrochemically active material of claim 12 having aBET surface area of 10 m²/g to 500 m²/g.
 18. The electrochemicallyactive material of claim 12 having a BET surface area of 20 m²/g to 100m²/g.